Automated guided vehicle mission reliability modelling using a combined fault tree and Petri net approach

Abstract ：Automated guided vehicles (AGVs) are being extensively used for intelligent transportation and distribution of materials in warehouses and autoproduction lines due to their attributes of high efficiency and low costs. Such vehicles travel along a predefined route to deliver desired tasks without the supervision of an operator. Much effort in this area has focused primarily on route optimisation and traffic management of these AGVs. However, the health management of these vehicles and their optimal mission configuration have received little attention. To assure their added value, taking a typical AGV transport system as an example, the capability to evaluate reliability issues in AGVs are investigated in this paper. Following a failure modes effects and criticality analysis (FMECA), the reliability of the AGV system is analysed via fault tree analysis (FTA) and the vehicles mission reliability is evaluated.

using the Petri net (PN) method. By performing the analysis, the acceptability of failure of the mission can be analysed, and hence the service capability and potential profit of the AGV system can be reviewed and the mission altered where performance is unacceptable. The PN method could easily be extended to have the capability to deal with fleet AGV mission reliability assessment.

Keywords:  Automated guided vehicles · Reliability · Petrinets · Fault tree analysis

1 Introduction

For intelligent transportation and distribution of materials in warehouses and/or manufacturing facilities, there has been in recent years the increasing use of automated guided vehicles (AGVs). Such vehicles travel along predefined routes

to deliver various tasks without the supervision of an onboard operator. As the AGV systems are getting larger and more complex, increasing the efficiency and lowering the operation cost of the AGV system have naturally become the first priorities, via investigating the design and control aspects of the AGV , by identifying new flow-path layouts including workstation layouts and developing advanced traffic management strategies, including vehicle routing and task assignment . Trenkle  introduced the safety requirements and safety functions for a decentralised controlled AGV system. Three major hazards, i.e. collision with a person, tilting over and falling down, were identified. The effects of the speed of AGVs, the braking distance and detection area requirements as well as the mean

time to dangerous failure and performance were analysed. Regarding failure response, Ebben developed a method for failure control management for a special case study of AGVs, an underground transportation system (with loaded and unloaded AGVs considered) . In the area of the reliability modelling of AGVs, Fazlollahtabar created a model to maximise the reliability of AGVs and minimise

their repair cost, and Tavana and Fazlollahtabar modelled the reliability of AGVs as a cost function to optimize time and cost objectives . However, fully understanding how AGVs can fail and the causes of such failures is still needed. Some progress was made in this area by Duran et al. who attempted to identify the basic failure modes of the light detection and ranging (LIDAR) system and the camera-based computer vision system (CV) on AGVs by using a combined approach of fault tree analysis (FTA) and Bayesian belief networks (BN). In the work, human injury, property damage and vehicle damage were defined as the top events in the fault tree. However, the research did not cover all components and subassemblies included in AGVs.

Modelling using Petri net (PN) has been becoming a common tool and popular research topic to evaluate reliability of a system or a mission. For example, Wu proposed an extended object-oriented Petri net model to analyse the reliability of a phased mission with common cause failures in 2015. On the other hand, from the aspect of industrial applications, Le and Andrews presented a wind turbine asset model to study the degradation, maintenance and inspection processes of different wing tunnel components based on PNs. However, to date the Petri net (PN) method has only been used as a mathematical tool to investigate route planning and control strategies for AGV systems. For example, Luo and Ni designed a programmable logical controller (PLC) using Petri nets to prevent collisions of vehicles in an AGV system and Nishi and Maeno proposed an approach to optimise the routing planning for AGVs in semiconductor fabrication bays.     However, to the best of the authors knowledge, PNs have rarely been applied to the study of the reliability of AGVs. In particular, their application to mission reliability. This is the aim of the work presented in this paper. In contrast to the combined approach of FTA and BN adopted in [18], the combined use of FTA and PNs adopted here enables not only the analysis of all failure modes of all the subsystems but also an analysis of the mission of the AGV. In addition, the PNs can be easily modified if the mission changed.